Typically, prior art security seals have employed metal tapes or wires whose ends are joined by one-time-use clasps, interlocking metal or plastic cups, or moldable material carrying an inscribed serial number or other type of identifiable mark. Some types of seals have, in addition, randomly inscribed marks or randomly distributed particles which are photographed prior to use and are re-photographed after the seal has been removed and disassembled. Still other seals are identified with the aid of ultrasonic scanners. Although these types of seals are generally effective, they cannot withstand sustained attach, may be counterfeited, are usually not reusable, and cannot be continuously monitored.
Fiber optic seals with juxtaposed and twisted ends are described in U.S. Pat. No. 3,854,792. In devices described in that patent, half of the joined end is illuminated, producing a unique pattern of light on the other half.